Many types of antennas are in wide use today throughout the communications industry. The antenna has become an especially critical component for an effective wireless communication system due to recent technology advancements in areas such as Personal Communications Services (PCS) and cellular mobile radiotelephone (CMR) service. One antenna type that has advantageous features for use in the cellular telecommunications industry today is the dual polarized antenna which uses a dipole radiator having two radiating sub-elements that are polarity specific to transmit and receive signals at two different polarizations. This type antenna is becoming more prevalent in the wireless communications industry due to the polarization diversity properties that are inherent in the antenna that are used to increase the antenna's capacity and to mitigate the deleterious effects of fading and cancellation that often result from today's complex propagation environments.
Dual polarized antennas are usually designed in the form of an array antenna and have a distribution network associated with each of the two sub-elements of the dipole. A dual polarized antenna is characterized by having two antenna connection terminals or ports for communicating signals to the antenna that are to be transmitted, and for outputting signals from the antenna that have been received. Thus the connection ports serve as both input ports and as output ports at any time, or concurrently, depending on the antenna's transmit or receive mode of operation.
An undesirable leakage signal can appear at one of these ports as a result of a signal present at the opposite port and part of that signal being electrically coupled, undesirably so, to the opposing port. A leakage signal can also be produced by self-induced coupling when a signal propagates through a power divider and feed network.
The measuring of leakage signals is illustrated in the conventional art of FIG. 1. A main transmission signal al can be inputted at port 35. This transmission signal al is propagated by the antenna elements 11 coupled to port 35 when these antenna elements 11 are operating in a transmit mode. An undesirable leakage signal b1 can be measured at port 35 as a result of the transmission signal al exciting portions of the feed network such as distribution network 15.
In another example, the undesirable leakage signal b1 can be measured at port 35 when a transmission signal a2 is inputted at port 40. The transmission signal a2 can excite portions of the feed network such as distribution network 17 which in turn, can excite antenna elements 11, 12 or distribution network 15 or both. It is noted that other leakage signals (not shown) may be measured at port 40 which are caused by transmission signal a2 itself or signals inputted at port 35.
A dual polarized antenna's performance in terms of it transmitting the inputted signal with low antenna loss of the signal, or of it receiving a signal and have low antenna loss at the antenna's output received signal, can be measured in large part by the signals' electrical isolation between the antenna's two connection ports, i.e., the port-to-port isolation at the connectors or the minimizing of the leakage signal b1. Dual polarized antennas can also have radiation isolations defined in the far-field of the antenna which differ from port-to-port isolations defined at the antenna connectors. The focus of this invention is not on far-field isolation, but rather with port-to-port isolations at connector terminals of a dual polarized antenna.
While a dual polarized antenna can be formed using a single radiating element, the more common structure is an antenna having an array of dual polarized radiating elements 10. In practice, both the transmit and receive functions often occur simultaneously and the transmit and received signals may also be at the same frequency. So there can be a significant amount of electrical wave activity taking place at the antenna connectors, or ports, sometimes also referred to as signal summing points.
The significant amount of electrical wave activity during simultaneous transmission and reception of RF signals can be explained as follows. Poor receive sensitivity, and poor radiated output, often results due to degraded internal antenna loss when part of one of the signals at one input port (port one) leaks or is otherwise coupled as a leakage signal to the other port (port two). Such leakage or undesired coupling of a signal from one port to the other adversely combines with the signal at the other port to diminish the strength of both signals and hence reduce the effectiveness of the antenna. When port-to-port isolation is minimal, i.e., leakage is maximum, the antenna system will perform poorly in the receive mode in that the reception of incoming signals will be limited only to the strongest incoming signals and lack the sensitivity to pick up faint signals due to the presence of leakage signals interfering with the weaker desired signals. In the transmit mode, the antenna performs poorly due to leakage signals detracting from the strength of the radiated signals.
Dual polarized antenna system performance is often dictated by the isolation characteristic of the system and the minimizing or elimination of leakage signals.
Conventional Isolation Techniques
One known technique for minimizing this leakage signal problem is by incorporating proper impedance matching within the distribution networks of the two respective signals. Impedance mismatch can cause leakage signals to occur and degrade the port-to-port isolation if (1) a cross-coupling mechanism is present within the distribution network or in the radiating elements, or if (2) reflecting features are present beyond the radiating elements. Impedance matching minimizes the amount of impedance mismatch that a signal experiences when passing through a distribution network, thereby increasing the port-to-port isolation.
In general, when impedance mismatches are present, part of a signal is reflected back and not passed through the area of impedance mismatch. In a dual polarized antenna system, the reflected signal can result in a leakage signal at the opposite port or the same port and it can cause a significant degradation in the overall isolation characteristic and performance of the antenna system. While impedance matching helps to increase port-to-port isolation, it falls short of achieving the high degree of isolation that is now required in the wireless communications industry.
Another technique for increasing the isolation characteristic is to space the individual radiating elements of the array sufficiently apart. However, the physical area and dimensional constraints placed on the antenna designs of today for use in cellular base station towers generally render the physical separation technique impractical in all but a few instances.
Another technique for improving an antenna's isolation characteristic is to place a physical wall between each of the radiating elements. Still another is to modify the ground plane 30 of the antenna system so that the ground plane 30 associated with each port is separated by either a physical space or a non-conductive obstruction that serves to alleviate possible leakage between the two signals otherwise caused by coupling due to the two ports sharing a common ground plane 30. These techniques can help in increments, but do not solve the magnitude of the signal leakage problem.
Still another conventional technique for improving the isolation characteristic of an antenna is to use a feedback element to provide a feedback signal to pairs of radiators in the antenna array. The feedback element can be in the form of a conductive strip placed on top of a foam bar positioned between radiators. While the conductors, according to this technique, can increase the isolation characteristic, the foam bars that support the conductive strips have mechanical properties that are not conducive to the operating environment of the antenna. For example, the foam bars are typically made of non-conducting, polyethylene foam or plastic. Such materials are usually bulky and are difficult to accurately position between antenna elements.
Additionally, these support blocks have coefficients of thermal expansion that are typically not conducive to extreme temperature fluctuations in the outside environment in which the antenna functions, and they readily expand and contract depending on temperature and humidity. In addition to the problems with thermal expansion, the support blocks are also not conducive for rapid and precise manufacturing. Furthermore, these types of support blocks do not provide for accurate placement of the conductive strips or feedback elements on the distribution network board.
Another problem with this conventional type feedback element is that the element is typically “floating” above its respective ground plane. That is, it is not connected to the ground plane or “grounded”. Such an ungrounded feedback system is susceptible to electrostatic charging. The electrostatic charging of these type conductive elements may attract lightning or currents that are formed from lightning.
Consequently, there is a need in the art for a method and system that facilitates the design of a dual polarized antenna system with a high degree of isolation between two respective antenna connection ports that more thoroughly cancels out any port-to-port leakage signals and at the same time, is conducive to high speed manufacturing and a high degree of accurate repeatability. There is also a need in the art for an antenna isolation method and system that can withstand extreme operating environments as a cellular base station antenna is subjected to, and one that is also designed to eliminate any potential problems that are a result from lightning or further leakage from electric charge build-up.